Recent development of materials with large second and third order nonlinear optical (NLO) effects has generated interest in having application to future commercial electronic and telecommunication systems. Areas of application include optoelectronic interconnects to replace metal interconnects in computers for increased signal propagation rates and reduced crosstalk. The performance of spatial light modulators, used in most optical computing systems, is at present limited by the uniformity, reproducibility and cost of the nonlinear optical materials (NLOM) used in their manufacture. Another application of NLOM is in memory systems. NLOM have potential application in memory systems of reducing random access times and cost per stored bit while simultaneously increasing storage density. Finally, NLOM are of potential utility in protective sensors for the human eye, as well as in such applications as rangefinders, visual sensors and cameras.
The ideal nonlinear optical material for most applications would have a very large nonlinear response, extremely low switching thresholds and rapid switching times. NLOM for commercial applications should be inexpensive, mechanically tough, and formable into thin films. The materials should also be resistant to laser irradiation, chemicals, and temperature changes. Recently, organic and polymeric materials have been of particular interest due to their promising potential applications in optical information processing and telecommunications. This interest has arisen from the promise of attractive combinations of optical, structural, and mechanical properties. Organic and polymeric materials can exhibit considerably high optical damage thresholds compared with inorganics because the former are not as susceptible to the formation of F-centers. In addition, the ability to prepare numerous derivatives of organics implies that properties can be tuned to meet specific requirements. Because of their processability into various forms, polymers seem particularly attractive for applications requiring nonlinear optics.
Although various organic and inorganic materials exhibit rather large second and third order optical nonlinearities, the performance of some of these materials is limited by the uniformity, reproducibility, and cost of the nonlinear optical materials used in their manufacture. Polymers with large nonlinearities are particularly attractive because of their processability into useful forms such as fibers and films.
Mechanical shear has been demonstrated to induce molecular alignment in several polymers. Alignment of liquid crystalline polymers in solution by application of mechanical shearing stresses is accompanied by the development of banded structures oriented perpendicular to the direction of shear. It is also well known that electric fields have pronounced effects on polymer morphology. Liquid crystals are particularly susceptible to electric field-induced suprastructure modulation due to their anisotropic molecular dimension and the presence of dipoles in their chemical structures. Not only might molecules align in an electric field, but also transitions between different liquid crystalline states may be observed. Many liquid crystals can undergo transitions between mesomorphic phases, and electric or magnetic fields can induce such transitions. For example, with cholesteric liquid crystals, electric fields tend to align the molecules in the field direction. However, at sufficiently high field strengths, a transition to nematic order can occur.
Biopolymers offer an attractive combination of structural and mechanical properties which supports optical nonlinearity. Poly-(.gamma.-benzyl-L-glutamate), PBLG, is known to form helical structures when dissolved in an appropriate helicogenic (helix-inducing) solvent. Further, the helices align in solution to form aggregates of higher order (liquid crystals). These structures, and films formed which capture these structures, are non-centrosymmetric, that is, they do not possess a center of inversion, and hence give rise to nonlinear optical responses. To the extent that both the degree of helicity and the extent of molecular alignment is increased, NLO responses are expected to increase.
Polypeptides which are characterized by helical structures are, at the molecular level, non-centrosymmetric. However, in randomly oriented polypeptide films, as might be obtained by solvent evaporation in the absence of an applied field, non-centrosymmetry is lost and the sample does not exhibit nonlinear optical properties such as second harmonic generation (SHG). Alignment of the molecular helices by application of an electric field perpendicular to the solution with simultaneous evaporation of the solvent yields SHG-active films with relaxation times reported to be at least six months.
Donald et al. (Polymer 24:155-159 (1983); J. Mat. Sci. 18:1143-1150 (1983)) reported banded structures formed by several thermotropic polymers oriented by shear at temperatures above their softening points. Similar structures were also noted in fibers drawn from rigid backboned polyesters above the softening points. Toth and Tobolsky (Polymer Letters 8:531-536 (1970) applied electric fields perpendicular to 15% solutions of PBLG in chloroform and noticed the emergence of dark fields in previously birefringent solutions. Upon slight shearing birefringence reappeared as multicolored bands perpendicular to the shear direction. Toth and Tobolsky (Polymer Letters 8, p. 531 (1970)) indicated that the disappearance of the birefringence was due to a perpendicular orientation of the solute molecules with respect to the plane of the film which reverted to an in-plane orientation upon shear. Similar banded structures have been observed upon application of a magnetic field of approximately 9600 Gauss oriented parallel or perpendicular to the surface of a 15% solution of PBLG in methylene dibromide.
Kiss and Porter (Mol. Cryst. Liq. Cryst. 60:267-280 (1980)) observed the transverse striations in a rheooptical study of sheared solutions of about 14 wt % PBLG in m-cresol. Kiss and Gabor indicate that molecules within the striae are oriented at 45.degree. to the direction of shear and suggest that the striae form planes about 10-30 microns in width which traverse the entire sample volume.